Colour and the Optical Properties of Materials
An Exploration of the Relationship Between Light, the Optical Properties of Materials and Colour
PROFESSOR RICHARD J. D. TILLEY Emeritus Professor, University of Cardiff, UK
Colour and the Optical Properties of Materials
Colour and the Optical Properties of Materials
An Exploration of the Relationship Between Light, the Optical Properties of Materials and Colour
PROFESSOR RICHARD J. D. TILLEY Emeritus Professor, University of Cardiff, UK This edition first published 2011 Ó 2011 John Wiley & Sons, Ltd Registered office John Wiley & Sons, Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, United Kingdom For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at www.wiley.com. The right of the author to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988.
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Library of Congress Cataloging-in-Publication Data
Tilley, R. J. D. Colour and the optical properties of materials : an exploration of the relationship between light, the optical properties of materials and colour / Richard J. D. Tilley. p. cm. Includes bibliographical references and index. ISBN 978-0-470-74696-7 (cloth) – ISBN 978-0-470-74695-0 (pbk.) 1. Light. 2. Optics. 3. Color. I. Title. QC355.3.T55 2010 535.6–dc22 2010025108
A catalogue record for this book is available from the British Library. ISBN 9780470746967 [HB] ISBN 9780470746950 [PB]
Set in 10/12pt Times Roman by Thomson Digital, Noida, India To Anne, for her continued help and support
Contents
Preface xv
1 Light and Colour 1 1.1 Colour and Light 1 1.2 Colour and Energy 3 1.3 Light Waves 5 1.4 Interference 7 1.5 Light Waves and Colour 9 1.6 Black-Body Radiation and Incandescence 10 1.7 The Colour of Incandescent Objects 13 1.8 Photons 14 1.9 Lamps and Lasers 16 1.9.1 Lamps 16 1.9.2 Emission and Absorption of Radiation 17 1.9.3 Energy-Level Populations 17 1.9.4 Rates of Absorption and Emission 18 1.9.5 Cavity Modes 21 1.10 Vision 23 1.11 Colour Perception 28 1.12 Additive Coloration 29 1.13 The Interaction of Light with a Material 33 1.14 Subtractive Coloration 37 1.15 Electronic ‘Paper’ 39 1.16 Appearance and Transparency 40 Appendix A1.1 Definitions, Units and Conversion Factors 43 A1.1.1 Constants, Conversion Factors and Energy 43 A1.1.2 Waves 43 A1.1.3 SI Units Associated with Radiation and Light 45 Further Reading 47
2 Colours Due to Refraction and Dispersion 49 2.1 Refraction and the Refractive Index of a Material 49 2.2 Total Internal Reflection 54 2.2.1 Refraction at an Interface 54 Contents viii
2.2.2 Evanescent Waves 54 2.3 Refractive Index and Polarisability 58 2.4 Refractive Index and Density 60 2.5 Invisible Animals, GRINs and Mirages 62 2.6 Dispersion and Colours Produced by Dispersion 65 2.7 Rainbows 68 2.8 Halos 75 2.9 Fibre Optics 75 2.9.1 Optical Communications 75 2.9.2 Optical Fibres 77 2.9.3 Attenuation in Glass Fibres 79 2.9.4 Chemical Impurities 80 2.9.5 Dispersion and Optical-Fibre Design 81 2.10 Negative Refractive Index Materials 84 2.10.1 Metamaterials 84 2.10.2 Superlenses 87 Further Reading 89
3 The Production of Colour by Reflection 91 3.1 Reflection from a Single Surface 92 3.1.1 Reflection from a Transparent Plate 92 3.1.2 Data Storage Using Reflection 94 3.2 Interference at a Single Thin Film in Air 94 3.2.1 Reflection Perpendicular to the Film 96 3.2.2 Variation with Viewing Angle 97 3.2.3 Transmitted Beams 98 3.3 The Colour of a Single Thin Film in Air 99 3.4 The Reflectivity of a Single Thin Film in Air 101 3.5 The Colour of a Single Thin Film on a Substrate 102 3.6 The Reflectivity of a Single Thin Film on a Substrate 104 3.7 Low-Reflection and High-Reflection Films 105 3.7.1 Antireflection Coatings 105 3.7.2 Antireflection Layers 106 3.7.3 Graded Index Antireflection Coatings 108 3.7.4 High-Reflectivity Surfaces 110 3.7.5 Interference-Modulated (IMOD) Displays 110 3.8 Multiple Thin Films 111 3.8.1 Dielectric Mirrors 111 3.8.2 Multilayer Stacks 113 3.8.3 Interference Filters and Distributed Bragg Reflectors 114 3.9 Fibre Bragg Gratings 115 3.10 ‘Smart’ Windows 119 3.10.1 Low-Emissivity Windows 119 3.10.2 Self-Cleaning Windows 121 3.11 Photonic Engineering in Nature 121 3.11.1 The Colour of Blue Butterflies 122 3.11.2 Shells 122 ix Contents
3.11.3 Labradorite 122 3.11.4 Mirror Eyes 125 Appendix A3.1 The Colour of a Thin Film in White Light 126 Further Reading 127
4 Polarisation and Crystals 129 4.1 Polarisation of Light 129 4.2 Polarisation by Reflection 131 4.3 Polars 135 4.4 Crystal Symmetry and Refractive Index 137 4.5 Double Refraction: Calcite as an Example 138 4.5.1 Double Refraction 138 4.5.2 Refractive Index and Crystal Structure 140 4.6 The Description of Double Refraction Effects 143 4.6.1 Uniaxial Crystals 143 4.6.2 Biaxial Crystals 144 4.7 Colour Produced by Polarisation and Birefringence 147 4.8 Dichroism and Pleochroism 149 4.9 Nonlinear Effects 151 4.9.1 Nonlinear Crystals 151 4.9.2 Second- and Third-Harmonic Generation 153 4.9.3 Frequency Mixing 155 4.9.4 Optical Parametric Amplifiers and Oscillators 156 4.10 Frequency Matching and Phase Matching 157 4.11 More on Second-Harmonic Generation 160 4.11.1 Polycrystalline Solids and Powders 160 4.11.2 Second-Harmonic Generation in Glass 160 4.11.3 Second-Harmonic and Sum-Frequency-Generation by 161 Organic Materials 4.11.4 Second-Harmonic Generation at Interfaces 162 4.11.5 Second-Harmonic Microscopy 162 4.12 Optical Activity 162 4.12.1 The Rotation of Polarised Light 162 4.12.2 Circular Birefringence and Dichroism 166 4.13 Liquid Crystals 168 4.13.1 Liquid-Crystal Mesophases 168 4.13.2 Liquid-Crystal Displays 169 Further Reading 173
5 Colour Due to Scattering 175 5.1 Scattering and Extinction 175 5.2 Tyndall Blue and Rayleigh Scattering 176 5.3 Blue Skies, Red Sunsets 178 5.4 Scattering and Polarisation 181 5.5 Mie Scattering 184 5.6 Blue Eyes, Blue Feathers and Blue Moons 187 5.7 Paints, Sunscreens and Related Matters 188 Contents x
5.8 Multiple Scattering 190 5.9 Gold Sols and Ruby Glass 191 5.10 The Lycurgus Cup and Other Stained Glass 193 Further Reading 195
6 Colour Due to Diffraction 197 6.1 Diffraction and Colour Production by a Slit 198 6.2 Diffraction and Colour Production by a Rectangular Aperture 200 6.3 Diffraction and Colour Production by a Circular Aperture 202 6.4 The Diffraction Limit of Optical Instruments 203 6.5 Colour Production by Linear Diffraction Gratings 205 6.6 Two-Dimensional Gratings 208 6.7 Estimation of the Wavelength of Light by Diffraction 210 6.8 Diffraction by Crystals and Crystal-like Structures 211 6.8.1 Bragg’s Law 211 6.8.2 Opals 213 6.8.3 Artificial and Inverse Opals 218 6.8.4 The Effective Refractive Index of Inverse Opals 221 6.8.5 Photonic Crystals and Photonic Band Gaps 223 6.8.6 Dynamical Form of Bragg’s Law 224 6.9 Diffraction from Disordered Gratings 225 6.9.1 Random Specks and Droplets 225 6.9.2 Colour from Cholesteric Liquid Crystals 228 6.9.3 Disordered Two- and Three-Dimensional Gratings 230 6.10 Diffraction by Sub-Wavelength Structures 231 6.10.1 Diffraction by Moth-Eye Antireflection Structures 231 6.10.2 The Cornea of the Eye 233 6.10.3 Some Blue Feathers 234 6.11 Holograms 235 6.11.1 Holograms and Interference Patterns 235 6.11.2 Transmission Holograms 235 6.11.3 Reflection Holograms 237 6.11.4 Rainbow Holograms 239 6.11.5 Hologram Recording Media 240 6.11.6 Embossed Holograms 242 Further Reading 243
7 Colour from Atoms and Ions 247 7.1 The Spectra of Atoms and Ions 247 7.2 Terms and Levels 252 7.3 Atomic Spectra and Chemical Analysis 254 7.4 Fraunhofer Lines and Stellar Spectra 255 7.5 Neon Signs and Early Plasma Displays 256 7.6 The Helium Neon Laser 259 7.7 Sodium and Mercury Street Lights 262 7.8 Transition Metals and Crystal-Field Colours 264 7.9 Crystal Field Splitting, Energy Levels and Terms 270 xi Contents
7.9.1 Configurations and Strong Field Energy Levels 270 7.9.2 Weak Fields and Term Splitting 271 7.9.3 Intermediate Fields 273 7.10 The Colour of Ruby 277 7.11 Transition-Metal-Ion Lasers 281 7.11.1 The Ruby Laser: A Three-Level Laser 281 7.11.2 The Titanium Sapphire Laser 282 7.12 Emerald, Alexandrite and Crystal-Field Strength 283 7.13 Crystal-Field Colours in Minerals and Gemstones 284 7.14 Colour as a Structural Probe 287 7.15 Colours from Lanthanoid Ions 288 7.16 The Neodymium (Nd3+) Solid-State Laser: A Four-Level Laser 290 7.17 Amplification of Optical-Fibre Signals 294 7.18 Transition Metal, Lanthanoid and Actinoid Pigments 295 7.19 Spectral-Hole Formation 297 Appendix A7.1 Electron Configurations 300 A7.1.1 Electron Configurations of the Lighter Atoms 300 A7.1.2 The 3d Transition Metals 301 A7.1.3 The Lanthanoid (Rare Earth) Elements 301 Appendix A7.2 Terms and Levels 302 A7.2.1 The Vector Model of the Atom 302 A7.2.2 Energy Levels and Terms of Many-Electron Atoms 304 A7.2.3 The Ground-State Term of an Atom 306 A7.2.4 Energy Levels of Many-Electron Atoms 306 Further Reading 307
8 Colour from Molecules 309 8.1 The Energy Levels of Molecules 309 8.2 The Colours Arising in Some Simple Inorganic Molecules 311 8.3 The Colour of Water 315 8.4 Chromophores, Chromogens and Auxochromes 316 8.5 Conjugated Bonds in Organic Molecules: The Carotenoids 317 8.6 Conjugated Bonds Circling Metal Atoms: Porphyrins and Phthalocyanines 319 8.7 Naturally Occurring Colorants: Flavonoid Pigments 323 8.7.1 Flavone-Related Colours: Yellows 323 8.7.2 Anthocyanin-Related Colours: Reds and Blues 324 8.7.3 The Colour of Red Wine 328 8.8 Autumn Leaves 332 8.9 Some Dyes and Pigments 333 8.9.1 Indigo, Tyrian Purple and Mauve 335 8.9.2 Tannins 337 8.9.3 Melanins 337 8.10 Charge-Transfer Colours 340 8.10.1 Charge-Transfer Processes 340 8.10.2 Cation-to-Cation (Intervalence) Charge Transfer 341 8.10.3 Anion-to-Cation Charge Transfer 345 8.10.4 Iron-Containing Minerals 346 Contents xii
8.10.5 Intra-Anion Charge Transfer 348 8.11 Colour-Change Sensors 349 8.11.1 The Detection of Metal Ions 349 8.11.2 Indicators 350 8.11.3 Colorimetric Sensor Films and Arrays 353 8.11.4 Markers 354 8.12 Dye Lasers 355 8.13 Photochromic Organic Molecules 358 Further Reading 361
9 Luminescence 363 9.1 Luminescence 363 9.2 Activators, Sensitisers and Fluorophores 365 9.3 Atomic Processes in Photoluminescence 368 9.3.1 Energy Absorption and Emission 368 9.3.2 Kinetic Factors 370 9.3.3 Quantum Yield and Reaction Rates 371 9.3.4 Structural Interactions 374 9.3.5 Quenching 374 9.4 Fluorescent Lamps 379 9.4.1 Halophosphate Lamps 379 9.4.2 Trichromatic Lamps 381 9.4.3 Other Fluorescent Lamps 382 9.5 Plasma Displays 383 9.6 Cathodoluminescence and Cathode Ray Tubes 385 9.6.1 Cathode Rays 385 9.6.2 Television Tubes 386 9.6.3 Other Applications of Cathodoluminescence 389 9.7 Field-Emission Displays 390 9.8 Phosphor Electroluminescent Displays 391 9.9 Up-Conversion 394 9.9.1 Ground-State Absorption and Excited-State Absorption 395 9.9.2 Energy Transfer 399 9.9.3 Other Up-Conversion Processes 401 9.10 Quantum Cutting 402 9.11 Fluorescent Molecules 405 9.11.1 Molecular Fluorescence 405 9.11.2 Fluorescent Proteins 407 9.11.3 Fluorescence Microscopy 409 9.11.4 Multiphoton Excitation Microscopy 410 9.12 Fluorescent Nanoparticles 411 9.13 Fluorescent Markers and Sensors 412 9.14 Chemiluminescence and Bioluminescence 413 9.15 Triboluminescence 416 9.16 Scintillators 416 Further Reading 418 xiii Contents
10 Colour in Metals, Semiconductors and Insulators 419 10.1 The Colours of Insulators 420 10.2 Excitons 421 10.3 Impurity Colours in Insulators 424 10.4 Impurity Colours in Diamond 424 10.5 Colour Centres 429 10.5.1 The F Centre 429 10.5.2 Electron and Hole Centres 430 10.5.3 Surface Colour Centres 434 10.5.4 Complex Colour Centres: Laser Action 434 10.5.5 Photostimulable Phosphors 435 10.6 The Colours of Inorganic Semiconductors 436 10.6.1 Coloured Semiconductors 436 10.6.2 Transparent Conducting Oxides 437 10.7 The Colours of Semiconductor Alloys 440 10.8 Light Emitting Diodes 441 10.8.1 Direct and Indirect Band Gaps 441 10.8.2 Idealised Diode Structure 443 10.8.3 High-Brightness LEDs 445 10.8.4 Impurity Doping in LEDs 446 10.8.5 LED Displays and White Light Generation 446 10.9 Semiconductor Diode Lasers 448 10.10 Semiconductor Nanostructures 449 10.10.1 Nanostructures 449 10.10.2 Quantum Wells 451 10.10.3 Quantum Wires and Quantum Dots 454 10.11 Organic Semiconductors and Electroluminescence 457 10.11.1 Molecular Electroluminescence 457 10.11.2 Organic Light Emitting Diodes 459 10.12 Electrochromic Films 463 10.12.1 Tungsten Trioxide Electrochromic Films 465 10.12.2 Inorganic Electrochromic Materials 467 10.12.3 Electrochromic Molecules 468 10.12.4 Electrochromic Polymers 468 10.13 Photovoltaics 471 10.13.1 Photoconductivity and Photovoltaic Solar Cells 471 10.13.2 Dye-Sensitised Solar Cells 472 10.14 Digital Photography 474 10.14.1 Charge Coupled Devices 474 10.14.2 CCD Photography 476 10.15 The Colours of Metals 477 10.16 The Colours of Metal Nanoparticles 478 10.16.1 Plasmons 478 10.16.2 Surface Plasmons and Polaritons 479 10.16.3 Polychromic Glass 481 10.16.4 Photochromic Glass 482 10.16.5 Photographic Film 484 Contents xiv
10.16.6 Metal Nanoparticle Sensors and SERS 486 10.17 Extraordinary Light Transmission and Plasmonic Crystals 487 Further Reading 488 Index 491 Preface
This book is concerned with colour. It is not primarily a textbook on optics, but focuses attention upon the ways that colour can be produced and how these ways govern device applications. However it is not possible to discuss colour without reference to numerous optical properties, so these, too, are explained throughout the text. Colour, though, remains the dominant theme. When writing about colour and colour production from a scientific point of view one is beset by a number of language conflicts, arising from the historical importance of the subject. Much of this confusion is due to the fact that the terminology has arisen gradually, as a result of historical experiences that scientists of the day found difficult to understand and interpret. Thus, diffraction, scattering, reflection and refraction can all be considered to be scattering of photons, and the variety of terms in use only confuses the modern reader. Indeed, the nature of light itself leads to problems. Is it a series of waves or a spray of bullet-like photons? A light wave can apparently pass through holes in a metal foil that are far smaller than its wavelength. How can this be? Is the light, instead, a series of photons that can do this, and if so, how big is a photon? Other similar difficulties exist. A decaying and glowing fungus exhibiting bioluminescence does not produce light by the same mechanism as a light-emitting diode (LED) using electroluminescence, although both are termed luminescence. The termination ‘-chromic’ suffers from the same lack of precision. Thermochromic molecules may or may not exhibit colour changes by the same mechanism as electrochromic thin films. The names do not supply any information about this. The units used in the measurement of light are equally confusing. This is because absolute measurements of energy, radiometric units, do not correspond to visual perception, measured in photometric units. Many of these questions are resolved in this book, particularly with respect to light and colour. The explanations are taken at as simple a level that will allow an appreciation of the topic. The book falls into three recognizable sections. Chapter 1 is introductory and covers ideas of light as rays, waves or photons. The emission and absorption of radiation is described, as is the difference in light from an incandescent source and light from a laser. Vision and the perception of colour (physiology or psychology), and related aspects are described in outline, as is the technical measurement of colour. These are specialist topics, and the information here is designed only to cover the need of subsequent chapters. Finally, the way in which light can interact with a material is summarized, as a prologue to later chapters. Chapters 2 6 explain optical phenomena mostly in terms of light waves. Colour is generated when light waves comprising all colours (white light) are subdivided physically into a series of smaller wavelength ranges (i.e. colours). Traditional divisions of the topic are retained, although there is little to choose, theoretically, between labelling a process scattering, diffraction or reflection. Because of this, there is sometimes an ambiguity as to where a particular topic should be placed. For example, fibre Bragg gratings might be treated as multiple reflectors or as diffraction arrays. Mie scattering can be regarded as diffraction. The layout adopted here is one that fits best with the explanations involved. Preface xvi
Chapters 7 10 require a photon explanation to account for colour production. Fundamentally, the absorption and emission of light from atoms, ions and molecules forms the central theme, and for this a quantum mechanical approach is needed. Many of these processes are widely exploited in displays. Although these are technologically complex and require considerable engineering skills in production, the way in which they produce colour is always based upon recognisable physical and chemical principles. Because of this, displays are introduced throughout the text in terms of the appropriate colour-generating mechanism, rather than as a separate section. The topics covered encroach upon physics, chemistry, biology, materials science and engineering and many aspects of these intertwined subject areas are touched upon. Students of all of these disciplines should find this book of relevance to some of their studies or interests. Readers who need more information can turn to the Further Reading sections at the end of each chapter. These include selected references to the original literature or substantial reviews and will allow them to take matters further. In addition, the website (www.wiley.com/go/ tilley colour) that accompanies this book contains exercises and numerical problems which have been provided to illustrate and reinforce the concepts presented in the text. All readers are encouraged to attempt them. There are also introductory questions that appear at the start of each chapter which are designed to stimulate interest. The answers to these are found in the Chapter itself. In addition, the answers to these introductory questions and all the other exercises and problems are to be found on the accompanying website. Unfortunately, some important light-related topics have been omitted. These include the important biological topics of photoperiodism inanimals and plants and photosynthesis. Although colour is ofimportance in these topics, the specialist knowledge here is biological rather than optical, and information in this field is best reserved for biological texts. It is a pleasure to acknowledge the considerable help and encouragement received in the preparation of this edition. The editorial staff of John Wiley & Sons have always given both assistance and encouragement in the venture. I am indebted to Professor D. J. Brown, University of California, Irvine, USA; Mr A. Dulley, West Glamorgan Archive Service, Swansea, Wales; Dr A. Eddington, Dr J. A. Findlay; Professor I. C. Freestone, University of Cardiff, Wales; Spectrum Technologies plc, Bridgend, Wales; Dr M. Sugdon, De La Rue Group; Dr R. D. Tilley, Victoria University of Wellington, New Zealand; Professor X. Zhang, University of California, Berkeley, USA; Dr P.Vukusic, University of Exeter, England; Dr G. I. N. Waterhouse, University of Auckland, New Zealand. All of them readily provided photographic material. To all of these I express my sincere thanks. Allan Coughlin gave encouragement and advice, and the members of staff of the Trevithick Library, University of Cardiff, Wales, were indefatigable in answering my obscure queries. Finally, my thanks, as always, are due to my wife Anne, who tolerated my hours reading or sat in front of a computer without complaint, and made it possible to complete this work.
Richard J. D. Tilley South Glamorgan May 2010 1 Light and Colour
. What is colour? . Why do hot objects become red or white hot? . How do e-books produce ‘printed’ words?
1.1 Colour and Light
Colour is defined as the subjective appearance of light as detected by the eye. It is necessary, therefore, to look initially at how light is regarded. In fact, light has been a puzzle from earliest times and remains so today. In elementary optics, light can usefully be considered to consist of light rays. These can be thought of as extremely fine beams that travel in straight lines from the light source and thence, ultimately, to the eye. The majority of optical instruments can be constructed within the framework of this idea. However, the ray concept breaks down when the behaviour of light is critically tested, and the performance of optical instruments, as distinct from their construction, cannot be explained in terms of light rays. Moreover, colour is not conveniently defined in this way. For this, more complex ideas are needed. The first testable theory of the nature of light was put forward by Newton (in 1704) in his book Optics,in which it was suggested that light was composed of small particles or ‘corpuscles’. This idea was supported on philosophical grounds by Descartes. Huygens, a contemporary, thought that light was wavelike, a point of view also supported by Hooke. Young provided strong evidence for the wave theory of light by demonstrating the interference of light beams (1803). Shortly afterwards, Fresnell and Arago explained the polarisation of light in terms of transverse light waves. However, none of these explanations was able to refute the particle hypothesis completely. Nevertheless, the wave versus particle theories differed in one fundamental aspect that could be tested. When light enters water it is refracted (Chapter 2). In terms of corpuscles, this implied a speeding up of the light in water relative to air. Thewave theory demanded that the light should move more slowly in water than in air. The experiments were complicated by the enormous speed of light, which was known to be about
Colour and the Optical Properties of Materials Richard J. D. Tilley 2011 John Wiley & Sons, Ltd Colour and the Optical Properties of Materials 2
gamma ultra- micro- radio rays X-rays violet infrared waves waves
10-14 10-12 10-10 10 8 10-6 10-4 10-2 1 102 wavelength (m)
vis ble
400 500 600 700 Wavelength / nm violet blue green yellow orange red Figure 1.1 The electromagnetic spectrum. Historically, different regions have been given different names. The boundaries between each region are not sharply defined but grade into one another. The visible spectrum occupies only a small part of the total spectrum
3 108 ms 1, and it was not until April 1850 that Foucault first proved that light moved slower in water than in air, and seemingly killed the corpuscular theory then and there. Confirmation of the result by Fizeau a few months later removed all doubt. Over the years the wave theory became entrenched and was strengthened by the theoretical work of physicists such as Fresnel, who first explained interference and diffraction (Chapter 6) using wave theory. Polarisation (Chapter 4) is similarly explained on the assumption that light is a wave. The wave theory of light undoubtedly reached its peak when Maxwell developed his theory of electromagnetic radiation and showed that light was only a small part of an electromagnetic spectrum. Light was then imagined as an electromagnetic wave (Figure 1.1). Maxwell’s theory was confirmed experimentally by Hertz, whose experiments led directly to radio. The problem for the wave theory was that waves had to exist in something, and the ‘something’ was hard to pin down. It became called the luminiferous aether and had the remarkable properties of pervading all space, being of very small (or even zero) density and having extremely high rigidity. Attempts to measure the velocity of the Earth relative to the luminiferous aether, the so-called aether drift, by Michelson and Morley, before the end of the nineteenth century, proved negative. The difficulty was removed by Einstein’s theory of relativity, and for a time it appeared that a theory of light as electromagnetic waves would finally explain all optical phenomena. This proved a false hope, and the corpuscular theory of light was revived early in the twentieth century, principally by Einstein. Since 1895, it had been observed that when ultraviolet light was used to illuminate the surfaces of certain metals, negative particles, later identified as electrons, were emitted. The details of the experimental results were completely at odds with the wave theory. The electrons, called photoelectrons, were only observed if the frequency of the radiation exceeded a certain minimum value, which varied from one material to another. The kinetic energy of the photoelectrons was linearly proportional to the frequency of the illumination. The number of photoelectrons emitted increased as the intensity1 of the light increased, but their energy remained constant for any particular light source. Very dim illumination still produced small numbers of photoelectrons with the appropriate energy.
1 The imprecise expression ‘intensity’ has largely been replaced in the optical literature by well defined terms such as irradiance (Appendix 1.1). The term intensity is retained here (in a qualitative way to designate the amount of light) because of the historical context. 3 Light and Colour
The explanation of this ‘photoelectric effect’ by Einstein in 1905 was based upon the idea that light behaved as small particles, now called photons. Each photon delivered the same amount of energy. If this was sufficiently large, then the electron could be ejected from the surface. The energy of each photon, E, was proportional to the frequency of the illumination, so that the photoelectron could be ejected when the frequency passed a certain threshold, but not before that point was reached. Thereafter, increasing the frequency of the illumination allowed the excess energy to be displayed as an increase in kinetic energy. The kinetic energy of the photoelectrons ejected from a metal under this hail of photons could then be written as:
1 mv2 ¼ E f 2 where f is known as the work function of the metal and is simply the energy required to liberate the electron from the metal surface. The intensity of the light simply indicated the number of photons arriving at the surface, so that the number of photoelectrons emitted is a function of irradiance, but the energy of these electrons is a function of the frequency of the radiation. Einstein thus rescued the wave theory from the dilemma of the luminiferous aether and then seemingly wrecked the self-same theory via his explanation of the photoelectric effect. At present, all experiments show that light and its interaction with matter (i.e. atoms) is best described in terms of photons. At its simplest level, the statistical behaviour of a large number of photons is then represented very well by an electromagnetic wave. That is to say, photons are the components of a light beam, whilst waves are a mathematical description of a beam of light. In this book, explanations are given in terms of the simplest approach that is in accord with the observations. For large-scale phenomena, such as the operation of a magnifying glass, it is adequate to use the idea of a ray of light. When objects having dimensions of the order of hundreds of nanometres are encountered it is necessary to consider light to be awave.Atomic processes require a photon approach. It needs tobe stressed that these are not different fundamentally. All are contained within the theory of optics available today, generally described as quantum optics or quantum electrodynamics. No matter how it is described, light has no colour as such. Light simply leaves the generating source, possibly interacts with matter in the course of passage and then enters the eye. Colour, or more accurately the perception of colour, is the result of an eye brain combination that serves to discriminate between light of different wavelengths or energies. In the following chapters, the production of light and its interaction with matter is discussed from the point of view of colour that of the original light source, and how this is modified by interaction with matter to generate new colours.
1.2 Colour and Energy
Colour is generated by interactions of light and matter atoms and molecules, or, more strictly, the electrons associated with these. If light is considered as an electromagnetic wave, then the energy density of the wave, which is the energy per unit volume of the space through which the light wave travels, is given by:
2 E ¼ e0ðE0Þ where e0 is the vacuum permittivity and E0 is the amplitude of the electric component of the wave. Classical optics, the interaction of light with a transparent solid, in the main, is concerned with scattering of the light. This leads to the phenomena of reflection, refraction and so on. In these processes, colour is produced by interaction between various light waves, and energy exchange considerations hardly matter. These aspects of colour formation are covered in Chapters 2 6. Colour and the Optical Properties of Materials 4
When light is absorbed by or emitted from a material, say a gemstone such as ruby, energy changes are paramount. In this case, light is best regarded as a stream of photons; the energy of each photon being defined as:
¼ n ¼ hc ð : Þ E h l 1 1 where n is the frequency of the equivalent light wave, l is the wavelength of the equivalent light wave, h is Planck’s constant and c is the velocity of light in vacuum. The absorption of light by isolated atoms or molecules involves a change in energy of the electrons surrounding the atomic nuclei. These occupy a series of atomic or molecular orbitals, each of which can be assigned a precise energy. The energies of the orbitals form a sort of ladder (with variable rung spacing) from low to high, each separated from the next by an energy gap. Electrons are fed into the orbitals from lowest to highest energy until all of the electrons have been allocated, leaving the extra outer electron orbitals empty. The total energy of all the electrons in the atom at modest temperatures is represented by an energy level called the ground state. The absorption of light will cause an electron to move from the low-energy ground state E0 to an empty orbital at a higher energy. The new energy situation is represented by an energy level at energy E1 (Figure 1.2a). (These energy levels and how they are enumerated will be described in detail in later chapters.) The relationship between the energy change DE and the frequency n or thewavelength l of the light absorbed is: